1. Field of the Invention
The present invention relates generally to a fault determining apparatus for an exhaust passage switching valve, and more particularly, to such an apparatus for determining a fault of an exhaust passage switching valve which switches an exhaust passage for exhaust gases discharged from an internal combustion engine from a main exhaust passage to a bypass exhaust passage having an adsorbent capable of adsorbing hydrocarbons and moisture contained in the exhaust gases, and vice versa.
2. Description of the Prior Art
Generally, in an exhaust system of a gasoline engine or the like, a three-way catalyst is arranged at an intermediate portion of an exhaust pipe in order to purify harmful substances (hydrocarbons, carbon monoxide and nitrogen compounds) in exhaust gases. However, since the three-way catalyst has not been activated, for example, immediately after a cold start of the engine (for example, for about 30-40 seconds from the start), the harmful substances may not be sufficiently purified, so that among the harmful substance, particularly hydrocarbons are emitted from the engine as they are as unburned combustible components. Therefore, in order to prevent emission of hydrocarbons in the air, there has been proposed an engine which incorporates not only the three-way catalyst but also an adsorbent, which is capable of adsorbing hydrocarbons, in the exhaust pipe. Such an adsorbent is arranged in a bypass exhaust passage branched from a main exhaust passage in which a three-way catalyst is arranged. In addition, a switching valve is arranged in the exhaust pipe for switching an exhaust passage for exhaust gases. This switching valve opens/closes the bypass exhaust passage in accordance with the state of the three-way catalyst to switch the exhaust passage from the main exhaust passage to the bypass exhaust passage, and vice versa.
Specifically, when the three-way catalyst remains inactive, for example, at a start of the engine, the switching valve opens only the bypass exhaust passage to introduce exhaust gases into the bypass exhaust passage and adsorb hydrocarbons in the exhaust gases to the adsorbent, thereby purifying the exhaust gases. On the other hand, when the three-way catalyst has been activated by a warm-up operation of the engine, the switching valve closes the bypass exhaust passage to pass the exhaust gases only to the main exhaust passage to purify the exhaust gases with the three-way catalyst. The adsorbent adsorbs hydrocarbons in a low temperature state (for example, below 100xc2x0 C.), and desorbs the once adsorbed hydrocarbons when it has been heated to a predetermined temperature or higher (for example, 100-250xc2x0 C.). The desorbed hydrocarbons are recirculated to the engine through an EGR pipe and so on.
While the switching valve normally operates as described above to permit appropriate purification of exhaust gases from the engine immediately after the start thereof, a fault in the switching valve, if any, disables appropriate purification of exhaust gases, so that it is necessary to monitor the switching valve for a fault. A fault determining apparatus for determining such a fault in a switching valve is known, for example, from Laid-open Japanese Patent Application No. 10-159544.
This fault determining apparatus has a temperature sensor arranged at a location downstream of an adsorbent in a bypass exhaust passage. For determining a fault in a switching valve during an operation of the engine, the fault determining apparatus forces the switching valve to perform a predetermined switching operation, and determines a fault in the switching valve based on a change in temperature of exhaust gases (hereinafter called the xe2x80x9cpost-adsorption exhaust gasesxe2x80x9d in this disclosure) downstream of the adsorbent, which is adsorbing hydrocarbons, before and after the switching operation. More specifically, when the engine is, for example, in a steady-state operating condition (for example, during idling), the switching valve is forcedly switched to cause a temporary change in the state of the exhaust passage from a state in which the bypass exhaust passage is closed and the main exhaust passage is opened to the reverse state, i.e., a state in which the bypass exhaust passage is opened and the main exhaust passage is closed. Then, the fault determining apparatus determines that the switching valve is faulty if a changing amount of the temperature detected by the temperature sensor respectively before and after the switching operation is equal to or less than a predetermined value. It should be noted that this determination takes advantage of the fact that the adsorption of hydrocarbons by the adsorbent involves an endothermic action.
As described above, the fault determining apparatus forces the switching valve to operate, after the start of the engine, when the three-way catalyst has been activated so that exhaust gases can be purified by the three-way catalyst and when the engine remains in a steady-state operating condition, to thereby open the bypass exhaust passage and close the main exhaust passage, and determines whether the switching valve fails. As such, the fault determining apparatus can determine a fault in the switching valve only when the engine is operating in a steady-state condition after the temperature of exhaust gases has been stabilized, but cannot directly determine whether or not the switching valve is actually operating under conditions in which the switching valve should essentially operate. In addition, since the detected temperature is generally slow in change, low in responsibility, and susceptible to the influence of various parameters including external air temperature and so on, an erroneous determination is likely to result when the fault determination of the switching valve is based on the detected temperature. Also, the fault determining apparatus must delay the execution of the switching valve fault determination until the engine enters the steady-state operating condition after the start, causing the fault determination to be time-consuming. Furthermore, the switching valve must be forcedly operated only for determining a fault in the switching valve in the steady-state operating condition in which the switching valve need not be essentially operated. For implementing this switching valve operation for the fault determination, a switching valve fault determining mode must be specially provided.
The present invention has been made to solve the problems as mentioned above, and its object is to provide a fault determining apparatus for an exhaust passage switching valve which is capable of directly, rapidly and properly determining a fault in an exhaust passage switching valve, under conditions such as immediately after the start of an internal combustion engine, in which the switching valve should be essentially operated, without the need for setting a special fault determining mode.
To achieve the above object, the present invention provides a fault determining apparatus for determining a fault in an exhaust passage switching valve for switching an exhaust passage of exhaust gases discharged from an internal combustion engine between a first exhaust passage having a three-way catalyst and a second exhaust passage branched off at a location downstream of the three-way catalyst of the first exhaust passage and having filled in an intermediate portion thereof an adsorbent capable of adsorbing hydrocarbons and moisture in the exhaust gases in accordance with an activated state of the three-way catalyst. The fault determining apparatus includes a humidity sensor arranged at a location downstream of the adsorbent in the second exhaust passage for detecting a humidity of the exhaust gases introduced into the second exhaust passage, and switching valve fault determining means for determining a fault in the exhaust passage switching valve based on a result of detection performed by the humidity sensor when the exhaust passage should have been switched to the second exhaust passage by the exhaust passage switching valve.
According to the foregoing configuration, when the three-way catalyst has not been activated, such as immediately after the start of the internal combustion engine, the exhaust passage switching valve switches the exhaust passage to the second exhaust passage, causing exhaust gases to flow into the second exhaust passage so that hydrocarbons and moisture in the exhaust gases are adsorbed by the adsorbent. On the other hand, when the three-way catalyst has been activated, the exhaust passage is switched to the first exhaust passage, causing the exhaust gases to flow only into the first exhaust passage so that the exhaust gases are purified by the three-way catalyst. Then, upon switching the exhaust passage to the second exhaust passage, when the exhaust passage switching valve is normally operating so that exhaust gases are flowing sufficiently into the second exhaust passage, hydrocarbons and moisture in the exhaust gases are adsorbed by the adsorbent to cause a gradual reduction in the adsorbing capacity of the adsorbent, resulting in a gradual increase, on the contrary, in moisture in the exhaust gases (post-adsorption exhaust gases), and accordingly a gradual increase in the humidity detected by the humidity sensor arranged at a location downstream of the adsorbent. On the other hand, upon switching to the second exhaust passage, if the exhaust passage switching valve is not normally operating so that exhaust gases are not at all flowing into the second exhaust passage or are flowing but not sufficiently, the value of humidity detected by the humidity sensor hardly increases or its increasing rate becomes smaller. In this way, since the result of the detection performed by the humidity sensor when the exhaust passage switching valve should have switched the exhaust passage to the second exhaust passage differs depending on whether or not the exhaust passage switching valve is normally operating, a fault in the exhaust passage switching valve can be determined based on the result of the detection.
Also, unlike the prior art, the fault determination for the exhaust passage switching valve is performed under the conditions in which the exhaust passage switching valve should be operated, making use of its operation, so that the fault determination for the exhaust passage switching valve can be directly, properly and rapidly carried out without the need for setting a special fault determination mode or waiting for the internal combustion engine to enter the steady-state operating condition. Further, since the humidity of exhaust gases is used as a parameter for determining a fault in the exhaust passage switching valve, the fault determination can be accurately and properly carried out. It should be noted that xe2x80x9ca fault in the exhaust passage switching valvexe2x80x9d as used in the disclosure refers to not only a fault in the exhaust passage switching valve itself but also a fault in the entire exhaust passage switching system including a driver for driving the exhaust passage switching valve, and so on.
Preferably, in one embodiment of the fault determining apparatus for an exhaust passage switching valve, the switching valve fault determining means determines a fault in the exhaust passage switching valve based on the result of detection performed by the humidity sensor when a predetermined time has elapsed from a start of the internal combustion engine.
According to the foregoing embodiment, since the fault determination is performed for the exhaust passage switching valve based on the result of detection by the humidity sensor when the predetermined has elapsed from the start of the internal combustion engine (hereinafter simply called xe2x80x9cat the startxe2x80x9d), the fault determination can be reliably carried out at a predetermined timing after the determining time has elapsed from the start, for example, by comparing the result of the detection with a predetermined reference humidity of exhaust gases which should be detected after the lapse of the predetermined time when the exhaust passage switching valve is normally operating.
Preferably, in another embodiment, the fault determining apparatus for an exhaust passage switching valve further includes atmospheric state detecting means for detecting an atmospheric state, fault determination execution deciding means for deciding whether or not a fault determination is performed for the exhaust passage switching valve by the switching valve fault determining means based on results of detections performed by the atmospheric state detecting means and the humidity sensor at the start of the internal combustion engine.
According to the foregoing embodiment, the fault determination execution deciding means decides whether or not fault determination is performed for the exhaust passage switching valve by the switching valve fault determining means based on results of detections performed by the atmospheric state detecting means and the humidity sensor at the start of the internal combustion engine. Since the degree of increase in the humidity of exhaust gases depends on the atmospheric state, for example, a saturated absolute humidity in the atmosphere at the start, and the humidity of the exhaust gases detected by the humidity sensor at the start, this may cause an error in the fault determination based on the result of the detection by the humidity sensor. Therefore, the foregoing embodiment can avoid such erroneous determination on a fault in the exhaust passage switching valve.
Preferably, in one embodiment of the fault determining apparatus for an exhaust passage switching valve, the adsorbent includes zeolite.
According to the foregoing embodiment, since the adsorbent made of zeolite is highly heat resistant and also less susceptible to deterioration, as compared with, for example, silica gel, active carbon, or the like used as an adsorbent, the humidity of exhaust gases introduced into the second passage will not excessively increase due to a deterioration of the adsorbent, but the adsorbent provides a stable moisture adsorbing characteristic, thereby making it possible to more properly carry out the fault determination for the exhaust passage switching valve.